Much research effort has been made into producing electronic paper as an electronic medium. Electronic paper requires the following characteristics: being reflective rather than emissive; high white (or off white) reflectivity and contrast ratio; allowing high definition images; memory effect (referred to at times as bistability); low-voltage drive capability; slimness; lightness; and low cost. In particular, electronic paper is required to have as excellent white reflectivity and contrast ratio as paper with respect to its display characteristics, and the development of a display device offering all of these characteristics is far from easy. The rapid advancements in printed electronics allow for the creation of low cost special purpose smart devices integrating sensing, logic, communication, storage/memory, and electro-optic effects into a new breed of functional media. To be effective as a display technology for functional media, a display must ideally be constructed using an all print process (that is a purely additive process), be readily conformable to various shapes such as toys or consumer electronics devices, survive hot lamination for smart cards and stored value cards applications. For applications such as smart packaging, the substrate used to create this electronic medium is the package itself (that is, not an appliqué) and the manufacturing process must as aligned with graphic printing technology (e.g. screen printing, flexography, gravure, inkjet). This means that designs can be realized based on real life precision, registration, and accuracy of printing equipment. It is highly desirable that this electronic medium must be met with a minimal impact on picture quality. Quality of the reflectance, hues of colors during coloring, hues of colors over time, color matching, switching speed for both coloring and bleaching must as good as possible. The requirements must also not restrict the type of devices that be created. One must also ensure that large devices, devices with large icons and segments and multi-color devices can be created.
Most display (whether reflective, emissive, or trans-reflective) technologies rely on a field effect to generate a change in the image. This field effect has to impact the entire color changing structure associated with the color changing elements. The color changing elements are often referred to as the coloring changing plane, or the front plane (even if not in the front of the display). The field generation system is often referred to as the backplane even though it might not be solely placed in the back of the display device. The field(s) generated can be used to generate the image (mostly in emissive displays where energy is constantly used to emit light) or to change the image (mostly in reflective displays). This architecture is applied to the different types of Cathode Ray, LCD, LED, OLED, electrophoretic, electro-wetting. Part of this front plane architecture has to thus be transparent (or nearly transparent) to ensure the user sees the color changes of the color changing plane. It is also the structure adopted by solar cell systems.
Displays/Systems based on chromic effects stand in sharp contrast with these traditional displays in that they do not require a field effect to change the color and thus are not constrained by this field encapsulation requirement of non-chromic displays. Some of the key chromic effects known are as follows:
Photochromism—color change caused by light;
Thermochromism—color change caused by heat;
Tribochromism—color change caused by mechanical friction;
Piezochromism—color change caused by mechanical pressure;
Solvatochromism—color change caused by solvent polarity;
Halochromism—color change caused by a change in pH. (See Vincent et al U.S. Pat. No. 6,879,424, Vincent and Flick U.S. Pat. No. 7,054,050 as a reference);
Electrochromism—color change caused by electrical charge;
Ionochromism—color change caused by ions (some ionochromic systems are at times referred to as electrochromic even though the change in color is created by the insertion or the extraction of cations/anions rather than the redox effect of adding or removing electrons.
Among those, only electrochromic system rely on the presence of charges and thus on the need to conduct and direct charges to a part of the core color changes. Compound devices can be created, of course, where an electrical system is used to trigger another effect, but they are combination devices such as Tatsuura et all in U.S. Pat. No. 7,463,400.
Legacy Architectures for Electrochromic Systems
Electrochromism has been used for mirrors, windows, light modulator and display/electronics paper systems (see P. M. S Monk, R. J. Mortimer, and D. R. Rosseinsky, Electrochromism and Electrochomic Devices, ISBN 978-0-521-82269-5, 2007).
Regardless of the class of devices, three architectures have been traditionally introduced. These are:
1) A sandwich architecture is discussed by Fitzmaurice et al U.S. Pat. No. 6,301,038 where two substrates are used. It also introduces the concept of high surface area nanoporous electrochromic films. An advanced design supporting reflective and emissive designs using a sandwich architecture is disclosed in Mizuno et al U.S. Pat. No. 7,184,191 where the working side has two electrodes, one that emits light, one that reflect light. The substrates used are covered with transparent conductors.
2) Pichot et al in U.S. Pat. No. 7,460,289, introduced a monolith (single substrate) structure where a single substrate is used. The counter electrode (aka COM electrode for common) is printed first on the substrate, then the separator, then the working electrode. The working electrode has a single non-patterned conductor buried in its structure. Improvement on this concept has been developed by Leyland et al in Patent Application PCT/US2008/065062. This architecture is referred to as COM on substrate.
3) Another single substrate monolith architecture, referred to as SEG on substrate (as in segmented electrode relating to the area of the working electrode that changes color through the redox process) is described in Briancon et al, PCT/US2009/056162. In this application, a conductor is applied directly between the substrate supporting the working (typically segmented, thus SEG) SEG electrode, and the SEG electrode.
A fourth type of architecture was recently disclosed in PCT/US2009/056162 In this invention, a porous substrate is used inside the structure itself, because it is porous, electrolyte permeates through it. While developed for displays applications, nothing precludes it from being used for the other classes of devices, once a porous substrate can be made transparent. This architecture is referred to as substrate as SEP (for separator).
Regardless of the specific cell architecture, there are common limitations related to the design of the working (SEG) electrode of electrochromic (and ionochromic) systems. Those limitations come, among matters, from the selection and use of conductors inside the working electrode, namely uniform near transparent conductors, as well as requirements on manufacturing that are not aligned with the most effective manufacturing techniques available for printed electronics. The current invention resolves these limitations by focusing the control of electron and ion motions through the working electrode and performing this control in a manner that is invisible to an end user of the system.
Transparent conductors used in the industry are generally in the form of indium tin oxide (In2□xSnxO3 or ITO), fluorine doped tin oxide (FTO) and doped zinc oxide (Aluminium doped Zinc Oxide). ITO has a yellow tint. Transparent semiconductors, such as Indium Gallium Zinc Oxide, can also be used. Transparent conductors tend to break down due to fatigue. ITO is known to show degradation with time when subject to mechanical stresses (see Wen-Fa Wu and Bi-Shiou Chiouy, “Mechanical and optical properties of ITO films with anti-reflective and anti-wear coatings” in Applied Surface Science, Vol. 115, Issue 1 May 1997, Pages 96-102.) This is a serious limitation for items such a smart labels or credit cards as these items are often bent during use. Dealing with this limitation is not an issue when dealing with television and computer displays, but it is for printed electronics. The current invention resolves this problem by enabling materials other than ITO to be used for many designs as well when using ITO, hiding the effect of the cracks to the end user of the systems.
Conductive Polymers can also be used as a transparent conductor. Most of them are derivatives of polyacetylene, polyaniline, polypyrrole or polythiophene. The most prevalent are is poly(3,4-ethylenedioxythiophene) (PEDOT) or poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) PEDOT: PSS. It should be noted that PEDOT:PSS exhibits electrochromic behavior (see Berggren et al U.S. Patent application No. 2007/0076287 A1 for more details). This means that PEDOT:PSS has limitations with respect to color performance in electrochromic materials that rely on another material for color changes. More recently, Carbon Nanotubes (CNT) have been added as an option to electrochromic display, albeit with a degradation of close to 30% in transmissivity (see Hrautyunyan U.S. Patent No. Application 2006/0284538). This is a major limitation with respect to optical quality, namely reflectance and contrast ratio.
Grid structures have been proposed as a replacement for ITO. The basic idea is to print a grid of a material inherently more conductive than ITO with very fine lines (optimally invisible to the human eye). Materials used have included silver, copper. Grids are extensively used for touch screen applications (albeit printed on the outside of the display structure). The electrical vs. optical trade-off of such structures is driven by the fill factor of such a grid. Most grid-based technologies available are not able to achieve an appropriate performance between conductivity and transparency.
Grid structures are not readily amenable to support a poorly conductive SEG material as lateral conductivity might not be high enough to move charges across wires being printed, and they are not amenable to being printed using screen-printing, a method widely used in printed electronics.
In order to improve the image quality, in non-chromic designs and most electrochromic displays, the transparent conductor deposited on the inside of the front substrate is very thin. It is also uniform across the entire surface area. This is needed to avoid changing the color of the color emitting or reflective element behind the conductor and to not create visible artifacts. This has two major limitations: 1) it will inherently cause an optical loss and at times the addition of a tint to the working electrode and 2) it is not amenable to patterning. This approach has been used in Fitzmaurice et al U.S. Pat. No. 6,301,038 where a transparent conductor is used to bring charges to a porous film including a n-type redox chromophore attached to nanoparticles of anatase TiO2. It is also used in Fitzmaurice U.S. Pat. No. 7,358,358. Optical quality and control of switching are limitations of these designs. The current invention removes these limitations by optimizing the structure of the conductor.
For the sandwich architecture, Maricle and Giglia, U.S. Pat. No. 3,844,636, describes an electrochromic mirror relying on the Frank-Keldysh or Platt effect (see J. of Chem. Phys. 34, pp. 862-863, 1961). The conductor is a single uniform material deposited with a constant thickness across the minor. Mirrors have a strict requirement for uniformity of performance, and that precludes moving away from this basic requirement. This conductor must however be near transparent so reflection from the back electrode is adequate. It is important to notice that in this design, the homogeneous conductive layer is in direct contact with the electrolyte of the cell. This creates compatibility issues that preclude the selection of specific metal-electrolyte combinations. The current invention removes these limitations on material choices.
Bennett et al. describes an ionochromic window in U.S. Pat. No. 5,466,577. A slotted continuous layer is used behind an ionochromic layer. This architecture has several limitations. It requires a gap between the coloring electrode and the front substrate to allow ionically conductive electrolyte to contact the electrochromic layer to allow the ions to pass through the entire electrode in order to allow for ionic based coloring. The conductive nature of the electrode is thus not geared to the management of charge motion (in either two or three dimensions) as indeed the potential applied is used to create an electrical field, which in turn triggers the motion of the ions to create the ionochromic coloring effect. Windows have a strict requirement for uniformity of performance and that precludes moving away from this basic requirement Of course, the conductor must be transparent. It is important to notice that in these designs, the homogeneous conductive layer is in direct contact with the electrolyte of the cell. The current invention does not have these limitations.
A single conductor, which is not placed first in the field of vision, has been introduced for COM on Substrate by Pichot in U.S. Pat. No. 7,460,289. The conductor is an non-patterned layer consisting of a homogeneous porous material printed/deposited with a constant thickness across the display. It describes the conductive structure to be larger than the area to be displayed.
It has the limitation of not supporting conductive tracks inside the display, managing the coloring of multiple segments inside a single display. Because the chromic layer is disposed on a conductor, images will not only be visible when the chromic material is in the colored state, but also when it is in the non-colored state This effect, which is called ghosting, is due to even slight differences in hue between the (non-colored) chromic layer, which forms the positive of the image, and the conductor, which forms the negative (i.e. the background). It also requires very pure electrolyte to be used to ensure high bistability. The current invention does not have these limitations.
The trade-off between electrical and optical quality of the working electrode (coloring electrode) has been dominated in legacy designs by the need to have a continuous transparent conductor film that is transparent enough for a viewer to observe the image created by the electrochomic area. Such thin conductor films possess an inherently high resistance and because of that a reduced current handling capability. Putting the conductor layer inside the working electrode also allows for displays with faster switching time for the same reflectivity if properly designed.
Color enhancement is readily achieved in a display. The introduction of a diffuser film on the outside of a reflective display is known to improve the perception of brightness. Tinting substrate (when placed in front the display) can be used. It has the disadvantage of potentially not being scratch-free and can create some parallax issues with the rest of the displays. It has the disadvantage to not be as roll-to-roll manufacturing friendly and is subject to manufacturing and usage scratches.
A method of color enhancement is discussed by Leibowitz in U.S. Pat. No. 3,944,333 where the dielectric separator between electrodes is filled with pigments that obscure the counter electrode and improve the contrast of the working electrode. This design has limitations with respect to the formulation of the dielectric separator, most notably because it requires additional processing to avoid separation and lumping between pigments, contributing to shorter shelf life for the ink. The current invention introduces patterned color matching components inside the working electrode.
Another method of color enhancement is discussed in Morrison and Jacobson U.S. Pat. No. 6,580,545. There, a white layer is applied at the bottom counter electrode (the non viewing side) of a multi-stack structure consisting of three electrochromic displays. It is used to provide a base color to the display and is thus not patterned. It is similar in principal (back electrode) to the enhancement discussed in Jung et at U.S. Patent Application Publ. No. 2008/0304142. These two designs have the same limitations as other legacy designs with respect to selection of materials for the working electrode, separator or electrolytes. It is also not patterned and cannot be used for color matching.
There is a need for a class of graphic displays to have a display that does not reveal the image to be displayed to the human eye until the image is activated (for instance a lottery ticket with a WIN vs. LOSE message) This is in contrast with numeric (typically 7 segment displays) and alphanumeric (13 segment displays) where the user knows a-priori the set of message that can be displayed.
There is also a need for lifetime and operation color integrity over time. This is a critical element to manage for certain applications, such as sensors where the user compares the hue and intensity of a colored chromic layer against a reference (printed on the same substrate or a card).
There is also the need for a display structure that shows words such as NO POWER when no power is being applied. This has to be achieved using no power.
Heretofore, display designs required the pattern of the electrochromic material to match exactly the image being presented to the end user. (See Coleman U.S. Pat. Nos. 5,500,759 and 6,582,509. Brabec et al. PCT Application EP2005/056014 for typical examples). This limitation results in the need for precise alignment/registration of the printing of chromic layer material. When looking at printing displays and systems that include displays using screen-printing, the alignment that can be achieved using a web sheet process is typically +−50 um. In contrast, the registration achievable using a roll-to-roll process is a not lower than +−120 um. The resolution achievable with screen-printing is about 100 um. Based on these capabilities, printing the maximum precision chromic layer is problematic for a roll-to-roll process, especially if the chromic layer is deposited on a conductor layer that has varying thickness or variation in surface energy. Legacy designs exhibit another related significant implication for electrochromic systems where multiple areas of the display are independently addressable, such as the 7 segments of a 7 segments digit. FIG. 1 illustrates a single structural substrate printed electrochromic (often referred to as monolith architecture) display structure. It is based on the designs covered by U.S. Pat. No. 6,870,657. This monolith electrochromic display structure (120) is typical of prior art designs. It is viewed from the top of the display through the top substrate (101). This substrate 101 includes flexible material such as PET, PETG, PEN, thin glass, bendable glass, or any other transparent material. On this substrate (101), a transparent conductor material (metal, organic, semiconductor) layer (102) is deposited on at least a part of the inside of the display. The deposition may be performed using a multiple of means such as printing, sputtering, ion beam deposition, etc. On the bottom interface of layer (102), a layer (103) of chromic material is deposited. The layer (103) can be patterned or un-patterned. Together, the transparent conductor and active layer form the so-called working electrode (107). All or part of this working electrode will change color saturation or hue during operation. There can be a plurality of such electrodes in a single display. A separator layer (104), akin to the dielectric of batteries, is placed next to layer (103) covering its entire area to insulate the working electrode from the counter electrode structure (108). This layer (104) is an ion conductive typically electrically insulating layer. The counter electrode is composed of two layers: A reservoir layer (105) and a conductive layer (106). The area of the charge reservoir layer (105) fits within the area of the insulation layer (104). The bottom conductor layer (106) is deposited below the reservoir layer. It covers the entire area of the charge reservoir layer (105). This layer (105) can be patterned. The display is fed through two conductors, one (109) for the working electrode, the other (120) the counter electrode. These conductors can be in the form of wires or simply conductive tracks printed on a structure. A graphic layer (111) may be printed on the outside of the top substrate. Layers (103), (104), (105), and (106) are permeated by an ion carrier electrolyte.
The present invention addresses the problems of legacy displays as described below.